Disk drives store digitized data on disks, commonly referred to as platters. Although other digital storage devices, such as solid state memory devices are available, disk drives are currently the best alternative for minimum cost per megabyte and maximum speed and power. They fall into three broad categories; namely, magnetic, optical and magnetic-optical drives. Except for their methods of storing and retrieving information, the three categories share the same basic structure.
In magnetic drives data is stored (written) and retrieved (read) from magnetic fields in coatings on disks. Two types of magnetic drives are in use. In the first type, commonly referred to as hard or Winchester drives, a stack of non-removable hard disks is mounted in a housing on a common spindle and spins at a very high speed. Some housings are hermetically sealed while others are vented to the atmosphere. Adjacent to the disks are, spaced apart, inductive type magnetic heads which write and read data on to the coatings of the disks. The heads fly in unison above the rapidly spinning disks and are attached to common actuators or actuator arms.
In the second type, commonly referred to as a floppy drive, magnetic heads contact magnetic surfaces of thin flexible disks. The flexible disks are packaged inside of hard jackets. Protective liners made of non-woven fibers wipe the disks to minimize wear and abrasion of the magnetic media by removing and trapping particulate matter. The flexible disk spins at a relatively slow speed, around 300-360 rpm to limit the wear of the disk. The 3½ inch disk drive and the higher speed Zip, SuperDisk and Sony HiFD drives are examples of floppy drives.
Hard drives are manufactured to a high degree of precision and are assembled in dust free “clean rooms”. The magnetic heads of the hard drives fly several micro inches above the disks on air bearings, generated by the rapid spinning of the disks. Flying head heights are proportional to the rotational speeds of the disks and are so minute that a single dust particle or fingerprint can disable a drive. The demands of a competitive marketplace have continuously increased storage capacities and have increased disk rotational speeds to 7200 rpm and above. The high disk rotational speeds have required fluidized bearings in hard drive and other motors to reduce friction. When a disk speed falls below its design limit, there is a danger of a head crash (contact) with a disk and or excessive head/disk wear. High disk speeds increase disk spindle and head wear and cause undesirable vibrations.
Although flying height is not a problem with floppy drives, head and disk wear are obvious concerns with magnetic and optical drives.
CD-ROM, DVD and DVD R/W drives are exemplary of optical disk drives. The storage capacity of optical drives is considerably greater than the magnetic disk drives. Digital information is written by laser beams in patterns of bumps and flats or patterns of reflective and non-reflective areas and read by reflecting laser beams off the digitized media to detectors. Most optical drives are not sealed and utilize a single one side digitized surface of a disk. Some optical drives are available with multiple disks mounted on carousels and optical drives are available with double-sided disks in sealed cartridges. The double-sided disks are manually flipped to read both sides. The third category, magnetic-optical drives, incorporate both magnetic and optical technologies.
Although air temperature, pressure, high humidity and condensation problems with magnetic hard drives have been largely eliminated by employing sealed magnetic hard drives, contamination and wear continue to limit the utility of magnetic and optical drives. It is generally accepted that all drives ultimately fail and computer users are urged to back up data on hard magnetic drives with files on floppy or CD disks. Drive failures are unpredictable and can be catastrophic because of their effects on computer controlled processes and/or losses of important data. Drive failures can be costly because of interruptions of procedures, such as data processing, accounting, etc.
Drive stiction and friction deteriorate drive performance. It has been observed that most disk and slider wear occurs below the take off speeds in hard disk drives. (See “Bhushan, B., Tribology and Mechanics of Magnetic Storage Devices”, Springer-Verlag, New York, 1996, pg. 474) Stiction also increases the loading of spindle and stepper motors. Stiction and friction also accelerate drive and disk wear and can produce hard drive crashes. Chemical contamination and corrosion degrade drive magnetic performance and cause mechanical as well as electronic component failures. Reduced magnetic performance can increase soft error rates.
Harmful contaminants include dust and small abrasive particles, chemical vapors, adhesives and lubricant and organic contaminant films. Outgassings of adhesives, plastic and rubber materials are known to produce corrosive chemical vapors.
A recent study, published in the STLE Tribology Transactions (Vol. 47, No. 1/January-April 2004, pp. 103-110) identified liquid like, highly viscous scattering sites in disk drives. The scattering sites, referred to as cloud condensation nuclei (CCN), are comprised of nanodroplet size hygroscopic inorganic salts and low molecular weight polar organic compounds. Smears of CCN nanodroplets on disk drive heads were common to a variety of drive lubricants and adversely affected the acoustic emissions (AE) and friction of the drives.
U.S. Pat. No. 6,548,173, incorporated herein by reference, discloses an ultra low friction (sometimes referred to herein as a near frictionless carbon “NFC” coating) and ultra low wear amorphous carbon diamond-like coating. Diamond, diamond-like and amorphous carbon coatings are known in the art for resisting mechanical wear, abrasion and chemical corrosion. They can be deposited on substrate materials by chemical vapor deposition processes at temperatures ranging from 700-1000° C.
Although the previous known carbon and diamond-like coatings are very hard and abrasion resistant, they contain large diamond grains and/or their non-diamond precursor materials between grains are very rough. When used in machining or under sliding wear conditions, they produce high frictional losses and severe wear on initial mating surfaces. The amorphous carbon and diamond-like coating in U.S. Pat. No. 6,548,173 is extremely smooth, has a high hardness and is resistant to corrosion. It is electrically insulating, and optically transparent to visible infrared and ultraviolet light. It is very low in friction and has a high resistance to wear. Friction tests at the Argonne National Laboratory of the patented amorphous carbon coating in an inert gas atmosphere showed an ultra low friction coefficient within a range of 0.001 to 0.007. The lower 0.001 value is about 20 to 100 times better than a Teflon or a polished fine grain diamond coating. Another benefit of the amorphous carbon coating is that it can be deposited at much lower temperatures (150° C.-500° C.) on a variety of substrates, including metals, ceramics, polymers, and plastics.
The results of the Argonne National Laboratory tests are shown in FIGS. 4 through 6. In FIG. 4, a brief CSEM (pin on disk friction) test was performed to illustrate the effect of applying nitrogen gas to the interface of a small diameter ball and a rotating disk in a small enclosed environment. Both ball and disk were coated with a near frictionless coating NFC6. At a 120 m point on the x-axis, nitrogen gas was applied through a small diffuser as friction increased throughout a purge phase. Friction increased up to a 700 m point (Part 2) where the gas (concentration) was available in sufficient quantity to cause the friction to drop dramatically to 0.04. Nitrogen gas was temporarily turned off at a 1200 m point (Part 3) to show the negative effect of no nitrogen. At a 1300 m point (Part 4) the nitrogen gas was reapplied and friction once again dropped quickly to 0.04 at which point the gas was turned off.
Additional Argonne National Laboratory test results of the NFC coating are shown in FIGS. 5 and 6. In the FIG. 5 test, an NFC coated Sapphire ball was run against an NFC coated disk. The test results revealed that the coefficient of friction of the NFC coating is ultra-low in a 100% dry nitrogen environment. In the FIG. 6 test, an NFC pin was run on an NFC disk. The test revealed a substantial difference in the coefficients of friction in dry air versus dry nitrogen.